In accordance with the phase separation method of particle production, for example for production of latex particles, the latex material is disposed in a solvent, which in turn is suspended in droplets in a fluid bath. The solvent and bath materials are chosen such that the solvent is slightly soluble in the fluid path, but the latex is essentially insoluble in the bath. Thus, as solvent diffuses into the fluid bath material, the latex is continuously redistributed and concentrated. When all of the solvent has diffused out of the latex droplets, a solid latex particle remains, the size of which depends on the amount of latex material in the original droplet.
The prior art teaches numerous methods whereby droplets of solvent used in the phase separation process are sized and processed such that particles of corresponding size and precision will be produced.
The prior art teaches at least one method which purports to produce monodispersed particles in the range of two to five micrometers in diameter, and larger, at a precision of at least 2% by volume. This method is described by M. J. Fulwyler et al. in an article entitled "PRODUCTION OF UNIFORM MICROSPHERES", Review of Scientific Instruments, 44, 1973. Similar techniques are set forth in U.S. Pat. No. 4,162,282 to Fulwyler et al. entitled "METHOD FOR PRODUCING UNIFORM PARTICLES", issued July 24, 1979 from an application filed Apr. 22, 1976. In accordance with the Fulwyler et al. techniques, a core liquid is injected into a moving sheath liquid. When combined, sheath and core are together formed into biphasic droplets as the fluids are jetted from a vibrating nozzle. The droplets are collected, and by stirring are held suspended in a catch liquid until the core and sheath liquids from each droplet have diffused into the catch liquid, leaving particles formed of the materials which were dispersed within the core.
It has been found that, in accordance with the Fulwyler et al. methods, the technique by which the fast moving droplets are collected and stirred is an important and indeed critical mechanism for the formation of uniform particles. The fast flowing sheath of fluid forms biphasic droplets which have considerable kinetic energy and momentum; depending on droplet size and velocity, there exists a varying degree of risk, nearly always substantial, that the droplets will experience shear forces of such intensity that they are broken apart during the collection process.
In a concurrently filed, copending application, assigned to the assignee hareof and entitled "APPARATUS AND METHOD FOR PROMOTING THE FORMATION OF MICROPARTICLES", we describe and claim a system for utilization of the core/sheath approach to the generation of microparticles, as taught by Fulwyler, but at faster rates, employing substantially reduced sheath and catch liquid volumes, eliminating the need for droplet charging, and employing respective nozzle sizes which obviate the danger of frequent clogging. In accordance with that copending, concurrently filed disclosure, the velocity and momentum of the droplets and surrounding sheath liquid are mechanically but safely taken up during the onset of the phase separation process, but prior to the ultimate collection of the forming and partially formed particles. More specifically, the momentum of the sheath stream and core fluid droplets is transferred to a surrounding medium in the presence of shear forces low enough to cause breakage of droplets. In one embodiment, the microdroplet jet containing the sheath fluid is directed onto and down a hydrophobic surface inclined at a very low angle (e.g. one to ten degrees) relative to the inclination to the jet. Kinetic energy of the sheath stream and core droplets is lowered by transfer of momentum to the hydrophobic surface, and, to a lesser extent, by frictional loss between the droplet flow and the surface.
That copending, concurrently filed application provides for collection of the "setting" but not completely formed microparticles as runoff from the surface, into an unspecified collection vessel. Conventionally, the collection, final formation, and separation process is conducted in a batch processing mode. The above captioned Fulwyler patent for example, provides for the spheres and sheath liquid to be dumped into a catch liquid basin, with the catch liquid being stirred or agitated until the core liquid of the uniform spheres has dissolved into the catch liquid, leaving spherical solid particles to be removed and concentrated by settling, filtering, centrifuging, or otherwise.
It is a primary object of the present invention to provide improved apparatus for collecting and promoting the formation of microspheres or microcapsules. Allied objects include provision for continuous, rather than batch mode processing, and for providing a high degree of control over all important aspects of the process.